1. Field of the Invention
This invention relates to an airtight envelope for an image display panel, and an image display panel and a method for producing the image display panel; and more particularly to an airtight envelope for an image display panel formed into an airtight structure by sealedly joining an anode substrate and a cathode substrate to each other by means of a sealing material mainly consisting of frit glass, and an image display panel including a field emission type cathode acting as an electron source and a method for producing the image display panel.
2. Discussion of the Background
A fluorescent display panel which has been conventionally known as one of display panels in the art is generally constructed into an airtight structure by sealedly joining upper and lower substrates to each other. More particularly, such a fluorescent display panel is so formed that two glass plates serving as upper and lower substrates are arranged so as to be opposite to each other and a glass bar of a rectangular shape in section acting as side plates is arranged in a frame-like manner between the glass plates so as to be positioned at a periphery thereof. Then, the glass plates and glass bar are weldedly sealedly joined to each other by means of a sealing material mainly consisting of frit glass comprising low-melting glass, resulting in an airtight envelope structure being provided.
Now, such an airtight envelope structure will be described hereinafter with reference to FIG. 8.
In FIG. 8, reference numerals 40 and 41 designate an anode substrate and a glass cover, which are arranged so as to serve as an upper substrate and a lower substrate, respectively. The substrates each are formed into a thickness sufficient to permit the airtight envelope to bear an atmospheric pressure. More particularly, they are formed into a thickness of 1.3 to 3.5 mm depending on a size of the package. In particular, when the fluorescent display panel is of the graphic type, the substrates are formed into a thickness of about 5 mm in the case that the panel is not constructed so as to facilitate formation and arrangement of spacers in a display area thereof.
The anode substrate 40 is provided on an inner surface thereof with a display pattern 42 formed of phosphors. Reference numerals 43 and 44 designate a grid electrode and a filamentary cathode (hereinafter referred to as "filament"), respectively.
A gap or space defined between the anode substrate 40 and the glass cover 41 in which the grid electrode and filament 44 are arranged is evacuated to a vacuum. For this purpose, a procedure is employed which comprises the steps of forming the display pattern 42 on the anode substrate 40 by patterning of phosphors, arranging the grid electrode 43 and filament 44 above the abode substrate 40, disposing side glass plates 45 on a periphery of the anode substrate 40 and then sealedly mounting the glass cover 41 through the side plates 45 on the anode substrate 40 as shown in FIG. 8.
The sealed joining is carried out by interposedly arranging a low-melting sealing material 46 such as powdered glass between each of the side glass plates 45 and each of the upper and lower substrates 40 and 41 and heating it to a temperature of about 500.degree. C. This causes the sealing material 46 to be melted to sealedly join the anode substrate 40 and glass cover 41 to each other, resulting in a sealed airtight envelope being formed. Then, the airtight envelope thus formed is evacuated to a vacuum.
There is also known another display panel in the form of a field emission display (hereinafter also referred to as "FED"), in which a gap between an anode and a cathode is formed into a dimension as small as, for example, 200 .mu.m. Thus, when the FED, as shown in FIG. 9, is to be constructed in such a manner that side glass plates 55 arranged in a frame-like manner are fixed to upper and lower substrates 50 and 51 by means of a sealing material 56 as in the fluorescent display panel described above, it is required to form the side glass plates 55 defining a gap between the substrates 50 and 51 into a thickness as small as about 180 .mu.m. Unfortunately, it is substantially impossible to execute such formation. Also, this requires that a layer of the sealing material is formed into a reduced thickness; therefore, air bubbles which are possibly formed in the sealing material layer cause airtightness in the envelope to be deteriorated.
To the problem described above, such an approach as shown in FIG. 10 is proposed. In the approach, the side glass plates 55 are arranged on an inner periphery of the sealing material layer 56. Unfortunately, the approach causes a portion of the FED in which the side glass plates 55 are arranged to be excluded from a display area, resulting in the display area being significantly reduced.
Also, in order to permit the FED to bear an atmospheric pressure, the substrates 50 and 51 are formed into an increased thickness. However, formation of the substrates into a thickness as large as about 5 mm causes the FED to be highly increased in weight as compared with other display panels such as a liquid crystal display panel and the like.
In a conventional image display panel (hereinafter also referred to as "FED") which includes an airtight envelope and a field emission cathode (hereinafter also referred to as "FEC") acting as an electron source, various ways are employed to arrange support members between an anode substrate on which a display section is provided and a cathode substrate on which a cathode is provided to keep a gap between the anode substrate and the cathode substrate substantially constant.
A first way is to laminatedly arrange frit glass. More particularly, screen-printing of a paste containing frit glass and calcination of the paste is repeated to form laminated frit glass, resulting in the support member being provided. Alternatively, a plurality of paste layers formed in turn by printing may be calcined in a lump. Also, the support member may be formed by printing a photosensitive glass paste on at least one of the substrates, forming it into a predetermined shape by photolithography and then calcining it.
A second way is to use photosensitive resin. More particularly, heat-resistant photosensitive resin such as polyimide or the like is deposited on at least one of the substrates by spraying or the like and subject to masked exposure, followed by development, leading to provision of the support member of a desired shape.
Unfortunately, the ways described above exhibit disadvantages when a graphic display exhibiting resolution as high as CRT is to be provided by the FED.
In general, it is required to arrange the support members while minimizing deterioration in efficiency with which the FED is evacuated and minimizing generation of residual gas in the FED. Also, when, for example, phosphor layers of luminous colors R, G and B which have a width of 100 .mu.m are to be arranged at pitches of 0.36 mm in the display section of the FED, it is required to set a surface of each of the support members contacted with the substrate at a width as small as 50 .mu.m or less in order to restrain the support member from interfering with displaying by the FED. However, in view of dielectric strength between the cathode and the anode and spreading of electrons emitted from the cathode, it is required to form each of the support members into a height as large as 0.1 to 0.3 mm. Also, an arrangement of each of the support members is limited to a position between picture cells.
Now, disadvantages of the first and second ways described above will be described hereinafter.
The first way described above has a disadvantage of causing frit glass formed by printing to be restricted to a height or thickness as small as tens um. More particularly, in order to form the support member into a desired height, it is required to repeat printing for lamination of frit glass. Also, a configuration of the support member which is increased in height as compared with a width thereof renders formation of the support member by lamination of frit glass troublesome. Further, formation of the support member by lamination of frit glass fails to provide the support member with a desired configuration and provide satisfactory positional accuracy.
Also, the first way requires calcination of the paste, so that sagging occurs on an upper surface of the support member to render formation of the support member into a uniform height difficult. Thus, some of the support members in the FED fail to function to support the substrates, leading to deterioration in pressure resistance of the FED. Further, calcination causes vehicle contained in frit glass to be discharged in the form of gas from the support member, so that roughness is produced on a surface of the support member. Also, the roughness causes residual gas to adhere to the support member.
In the second way described above, a film formed by spraying is limited to a thickness as small as 100 .mu.m. Therefore, when it is desired to form the support member into a height exceeding 100 .mu.m, spraying must be repeated. Also, it causes emission of gas because of using an organic material.
In addition, use of the photosensitive glass paste in the first way and that of the photosensitive resin in the second way require formation of phosphor layers on the anode conductor subsequent to formation of the support members on the substrates. Unfortunately, formation of the phosphor layers is highly troublesome because it must be separately carried out for every phosphor.